Fig. 3.7-1. Longitudinal section of the shoot apical meristem of Coleus. The shoot apical meristem produces all the cells of the shoot, and the root apical meristem produces all those of the root. Thus most cells start out as small, more or less cuboidal cells not much larger than a nucleus, then the cells grow to their mature size and shape. The larger cells at the bottom center of the micrograph will develop into very large pith parenchyma cells, and prominent intercellular spaces will form as the cells round up and push slightly apart from each other. The cells along the surface of the meristem will differentiate into epidermis cells, secreting cutin to their outer, exposed side (Fig. 3.4-1). Some meristem cells will develop as elongate sieve tube members (Fig. 3.5-1) or fibers, and some cells might develop “arms” like those of cattails (Fig. 3.3-6). It is important to remember that as long as the cells are growing, the primary walls can be pushed into some unusual shapes, but they cannot pulled back – plant cells are not as free to change shape as animal cells are.